Not a single person went topless after inhaling unknown gas (5)
I believe the answer is:
ozone
'gas' is the definition.
(I've seen this before)
'not a single person went topless after inhaling unknown' is the wordplay.
'not a single person' becomes 'noone' (I've seen this before).
'went topless' means to remove the first letter.
'after inhaling' indicates putting letters inside.
'unknown' becomes 'z' (algebra).
'noone' with its initial letter taken away is 'oone'.
'oone' going around 'z' is 'OZONE'.
(Other definitions for ozone that I've seen before include "invigorating British air" , "Gas forming screen against ultraviolet radiation in the stratosphere" , "Gas in upper atmosphere" , "Form of atmospheric oxygen" , "A gas; threatened layer in air" .)